Simple equations for evaluating group factors of pile foundation for two-layered soil have been already proposed by the authors. In this paper, we improve the design equation for horizontal group factor, considering of the ratio of shear wave velocities of two-layered soil. As a result, the accuracy of horizontal group factor in the case of actual condition can be improved. In addition, the horizontal interaction factors for two-layered soil are proposed here. The horizontal group factor in the case of irregular disposition of piles can be precisely evaluated by using these interaction factors.
